1. Field Of The Invention
This invention relates to a process for concurrently producing propylene and methyltertiarybutyl ether using catalytic dehydrogenation and etherification reaction zones.
2. Brief Description Of The Prior Art
In U.S. Pat. No. 4,118,425, a process is disclosed for preparing the methyl ether of tertiary butanol by reacting methanol and isobutene. This general process is otherwise well known and is also disclosed in other patents. In U.S. Pat. No. 4,118,425, normal butane is isomerized to produce an isomate which contains isobutane, and normal butane. This isomate is dehydrogenated to form predominantly isobutene and normal butane by passing the isomate over a chromium-on-alumina catalyst where approximately half of the normal butane in the stream is converted to butenes. The dehydrogenate is then reacted with a water soluble alkanol, such as methanol, in the presence of an etherification catalyst. A reaction product results which contains the product ether, such as methyltertiarybutyl ether, n-butane and excess of the water soluble alcohol which has not entered into the etherification reaction. The effluent from the etherification reaction will also contain some linear butenes.
Water-soluble alcohol can be easily separated from the normal butane and from the product ether. Components of the etherification reactor effluent such as butene-1, cis-trans-butene-2, isobutane and normal butane pass through the etherification zone as inerts--that is, they are not converted to other products during etherification. The ether product is thus mixed with normal butane and isobutane.
Separation of the desired ether product from the other compounds with which it is mixed in the etherification reactor effluent can be effected by fractional distillation. The relatively heavy ether product is recovered from the bottom of the fractionation tower; the overhead from the tower includes normal butane and isobutane. The patentee states that overhead from the distillation may be recycled, or it may be withdrawn from the process, depending on the needs of the system. In either case, it does not appear that a separation of butane from isobutane is attempted, nor is there any effort made to isolate or remove linear butenes from the total effluent product from the etherification reactor. If the isobutanes are recycled, it appears from the disclosure of the patent that the n-butane is also recycled. It is indicated in this patent that it is desirable to recycle isobutane, and to include a certain amount of linear butane material in the charge stock to the dehydrogenation reactors. No concern is manifested for any build up of normal butane in the course of continuously operating the described process.